The separation of hydrogen isotopes, especially the preparation of relatively pure heavy water, deuterium oxide (D.sub.2 O), is of great importance to the nuclear industry. Historically the Girdler-Sulfide process involving isotopic exchange between hydrogen sulfide and water has been by far the most important means for heavy water production. Although a process for separating hydrogen isotopes by isotopic exchange between hydrogen and water has important advantages over the Girdler-Sulfide process, the successful achievement of a water-based exchange process has remained an elusive goal. Recent developments have made such a process increasingly feasible and renewed hopes for its eventual use. Butler and coworkers have extensively reviewed recent progress in this area: J. P. Butler, Separation Science and Technology, 15(3), 371 (1980); Canadian Patent No. 1,063,587; J. P. Butler, J. H. Rolston, and W. H. Stevens, "Novel Catalysts for Isotopic Exchange Between Hydrogen and Liquid Water", ACS Symposium Series, No. 68, SEPARATION OF HYDROGEN ISOTOPES, American Chemical Society (1978); see also J. H. Rolston et al. in Catalysis on the Energy Scene, S. Kaliaguine and A. Mahay, editors, Elsevier Science Publishers (1984). Although we provide an abbreviated summary below, the interested reader should consult these references for more detailed information.
The exchange between gaseous hydrogen and liquid water is known to be catalyzed by many metals. The exchange rate of the overall process is limited by the solubility of hydrogen in water, since the exchange rate at the interface of phases is quite small. This solubility limitation has been circumvented by metal catalyzed vapor phase isotope exchange between hydrogen and water followed by vapor-liquid exchange between water where the two stages are physically separate to ameliorate the rapid deactivation of metal catalysts by liquid water, but the resulting process remained too expensive to be commercially competitive.
Using the same basic approach of metal catalyzed vapor phase isotope exchange between hydrogen and water followed by vapor-liquid exchange between water phases, the next development was that of hydrophobic catalysts. Because of their hydrophobic character these catalysts were not as prone to deactivation by liquid water as had been the prior art catalysts. The hydrophobic catalysts could be used as a fixed bed in a trickle bed operation with liquid water and the gaseous hydrogen flowing through the bed countercurrently, where isotope exchange occurred between hydrogen and the water vapor arising from the partial pressure of liquid water at the exchange temperature. Continued research at the Chalk River Nuclear Laboratory of Atomic Energy of Canada Limited led to successive improvements culminating in a catalyst of platinum and carbon "wetproofed" by bonding to poly(tetrafluoroethylene), (PTFE), where the hydrophobic PTFE layer prevents wetting of the catalyst surface in water.
Catalysts based on platinized carbon have the great disadvantage of being pyrophoric and combustible. What is needed is an active, noncombustible, hydrophobic, acid stable catalyst support with good thermal stability. Especially for the separation of hydrogen isotopes in the trickle bed process previously referred to, it is desirable that metal loading be at least 8 weight percent. However, merely having a high metal loading by itself is insufficient, for it is necessary to have good platinum dispersion, preferably as a monolayer (100% dispersion), but with at least 60% dispersion.
Silicalite is a hydrophobic molecular sieve having properties as a support quite well suited to the process under consideration. Wanke et al. in U.S. Pat. No. 4,536,488 have described platinum on silicalite catalysts for the isotope exchange in question and made several significant observations. Although they were able to prepare highly loaded (12%) platinum on silicalite, the exchange rates using this catalyst were significantly lower than platinized carbon with similar loading owing to a relatively low platinum dispersion on the silicalite support. This observation led the patentees to investigate different procedures for metal impregnation and they described a procedure affording highly dispersed (93-110%) platinum on silicalite with loadings at 5.9-7.4 weight percent platinum. A peculiar trait of their method, as shown by the data in their Table 3, is that platinum loading is virtually independent of the amount of platinum offered to the silicalite; increasing the amount of platinum offered by 8 fold increased the platinum loading only from 5.9 to 7.4 weight percent. Their data also permit the fair inference that a loading greater than 7.5 weight percent platinum is not possible by their method.
Our invention is a method of preparing catalytic composites of noble metals deposited on silicalite where the composite contains at least 8 weight percent of a noble metal, or some combination of noble metals, with at least 60% dispersion. Our invention affords a catalyst which is quite active in the aforementioned isotope exchange process and which has a high useful lifetime without being combustible or pyrophoric, thereby representing a significant advance in this art. In another aspect our invention is an improved isotope exchange process, where the improvement consists of the use of the catalyst of our invention.